Wt is a C++ library for developing Web
applications with an API that is widget-centric
and inspired by existing C++ graphical user
interface APIs. To the developer, it offers
abstraction of Web-specific implementation details, increasing the accessibility and portability. Under the hood, the library uses the latest techniques (HTML5, Ajax, WebSockets) to handle user events and update the Web page.

GTK, which stands for the Gimp ToolKit, is a library for creating graphical user interfaces. It is designed to be small and efficient, but still flexible enough to allow the programmer freedom in the interfaces created. GTK provides some unique features over standard widget libraries.

The Xmt Library provides nine custom widgets and
over 260 convenience functions designed to make
Motif user interface development easier for
beginning and advanced programmers. It provides
an alternative tool chain that avoids the use of
GUI builders. The essence of this approach is to
program from the resources file using an extended
vocabulary for automatic creation of the widget
hierarchy, dynamic layout, and to provide a rapid
prototype environment.

Tkinter is Python's de facto standard GUI (Graphical User Interface) package. It is a thin object-oriented layer on top of Tcl/Tk. To use Tkinter, you don't need to write Tcl code, but you will need to consult the Tk documentation and occasionally the Tcl documentation (since Tk's low-level event handling mechanism is considered part of Tcl).